1. Field of the Invention
The present invention relates to an apparatus for monitoring an inside of a manhole, and more particularly, to an apparatus for monitoring an inside of a manhole in which the inside of the manhole is monitored using a simple communication and power supply unit.
2. Description of the Related Art
A conventional apparatus for monitoring an inside of a manhole includes power cables 12-1 and 12-2 which are underground transmission lines installed inside a manhole 11, a connecting unit 13 for connecting the two corresponding power cables 12-1 and 12-2 to each other if necessary, and two covers 14 and 15 for closing a ground opening 11-1 of the manhole 11, as illustrated in FIG. 1. The apparatus for monitoring an inside of a manhole further includes a sensor 16 for collecting maintenance information about the corresponding power cables 12-1 and 12-2, a circuit unit 17 for processing a signal of the maintenance information collected by the corresponding sensor 16, a predetermined small power type wireless transmitter 18 prescribed by the Propagation Law, a transmission antenna 18-1 for transmitting a wireless signal of the corresponding wireless transmitter 18, a reception relay antenna 21 installed on a bottom surface of the second cover 15 and receiving a wireless signal of the corresponding wireless transmitter 18, an impedance matching circuit unit 22 which is installed on a top surface of the second cover 15, whose input terminal being connected to the corresponding reception relay antenna 21 and which impedance-matches the corresponding reception relay antenna 21 and the corresponding first cover 14, and an electrode 23 connected to an output terminal of the impedance matching circuit unit 22.
An outer diameter of the first and second covers 14 and 15 is about 700-900 mm, respectively. The covers 14 and 15 have a double cover structure comprising the outer first cover 14 and the inner second cover 15 for preventing submerge. A distance between the first cover 14 and the second cover 15 is generally 200-300 mm.
A plurality of sensors 16 are installed in the manhole 11 so as to measure several physical values which will be used as maintenance information in the manhole 11. Here, only one sensor 16 is illustrated.
The circuit unit 17 and the wireless transmitter 18 are fixed on an inner wall 11-2 of the manhole 11. The transmission antenna 18-1 may be the most common rod antenna.
The reception relay antenna 21 may be configured in a general die pole shape. A direction of the reception relay antenna 21 is parallel to the transmission antenna 18-1 of the wireless transmitter 18.
The electrode 23 has a surface area having a proper size so as to form an electrostatic capacity between the electrode 23 and the first cover 14 and may be formed of copper or copper alloy of brass etc., for example.
An operation of the conventional apparatus for monitoring an inside of a manhole having the above structure will now be briefly described.
First, the maintenance information collected by the sensor 16 is transmitted to the wireless transmitter 18 via the circuit unit 17 and is sent as a wireless signal by the corresponding wireless transmitter 18 via the transmission antenna 18-1.
At this time, since a wireless signal cannot be radiated to an outside of the manhole 11 only via the wireless transmitter 18, transmission power of the corresponding wireless signal is extracted by the reception relay antenna 21 installed on a bottom surface of the second cover 15 of the corresponding manhole 11 and maximum power is rapidly supplied to the corresponding first cover 14 through an electrostatic capacity between the electrode 23 connected to the corresponding impedance matching circuit unit 22 and the bottom surface of the first cover 14 of the corresponding manhole 11.
As such, the first cover 14 serves as an antenna. Thus, the wireless signal is radiated to the outside via the first cover 14. The corresponding wireless signal is received by a ground wireless receiver 20 so that circumstances in the manhole 11 can be recognized. At this time, the corresponding wireless receiver 20 may be installed on the ground or may be held by a person in charge of maintenance.
However, in the conventional apparatus for monitoring an inside of a manhole described above, a unit for connecting a signal with an outside of the manhole for wireless communication is needed, a data transceiver is also needed outside the manhole and a special supply source for supplying power required for an apparatus is also needed. Thus, a power line should be drawn out from the outskirts of a city and a structure of the apparatus is complicated and reproducibility is lowered.